DESCRIPTION (Applicant's Abstract): Apoptotic T cells must be cleared efficiently by macrophages to prevent tissue damage, but ingestion of apoptotic cells causes macrophages to downregulate their own production of proinflammatory cytokines such as TNF and IL-8 and of chemokines. Hence, while ingesting apoptotic T cells during resolving pneumonia is beneficial, the same process could be immunosuppressive if pulmonary alveolar macrophages ingest dying T cells before or during encounters with pathogens. Indeed, studies funded by this project made the novel observation that many apoptotic lymphocytes are found in the lungs of mice. Thus, the lungs, a mucosal surface frequently exposed to pathogens, present a unique challenge in regulating macrophage clearance of apoptotic T cells while maintaining host defense. It is likely that this challenge is relevant both to the normal state, in which single alveolar macrophages encounter isolated apoptotic T cells, and when larger numbers of T cells die (e.g., following acute viral pneumonias and in chronic HIV infection). New preliminary data from this project suggest that ingestion of apoptotic T cells by lung macrophages is regulated, as an evolutionary adaptation, to minimize the immunosuppressive effect that could otherwise result at this site of frequent pathogen exposure. The goal of this project is to define the molecular basis and significance of regulated phagocytosis of apoptotic T cells in the lungs. It will test the following hypotheses: that downregulated phagocytosis of apoptotic T cells by resident murine alveolar macrophages results both from altered adhesion of apoptotic T cells and from altered signal transduction relative to control peritoneal macrophages; that effective clearance of apoptotic T cells during resolving lung inflammation depends on acquisition of an ingesting phenotype, probably mostly by differentiation of recruited monocytes by inflammatory cytokines; and that ingestion of apoptotic T cells carries a risk of impaired lung host defense against bacterial and fungal pathogens. Both primary resident alveolar and peritoneal macrophages from normal mice, and two immortalized murine macrophage cell lines (MH-S and J774A.1) will be used. Techniques will include static adhesion and phagocytosis assays, specific enzyme inhibitors, immunoprecipitation, Western blotting, flow cytometry, and use of in vivo murine models of fungal (Cryptococcus neoformans) and bacterial (Staphylococcus aureus) pneumonia. It is anticipated that the results will provide important new information about immunoregulation that will be relevant to viral, bacterial, and fungal infections in normal and immunocompromised hosts, development of autoimmunity and lung fibrosis.